Systems, methods, and circuits for driving large off-chip loads

ABSTRACT

A method and system to drive large off-chip loads, such as, for example, laser diodes, wherein the system includes an integrated circuit coupled to an external differential diode load. Alternatively, the external diode load may be driven single-ended. The integrated circuit includes a data buffer device and a clock buffer device. The integrated circuit also includes a multiplexer device coupled to the clock buffer device configured to multiplex a data input signal and a clock input signal received at respective inputs of the integrated circuit. If the external diode is single-ended, the data input signal is transmitted to the data buffer device, which is then used solely to drive the diode load. If the diode load is differential, the data buffer device receives the data input signal. At the same time, the multiplexer device receives both the data input signal and the clock input signal and selects the data signal to drive the clock buffer device. For a diode load being driven differentially, the outputs of the two buffer devices are merged together externally through the use of a power-combining network, which includes external or off-chip transmission lines, which carry a respective output signal for the data buffer device and the clock buffer device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 11/367,253 filed Mar. 2, 2006, now U.S. Pat. No. 7,885,300 entitled “Methods and Circuits for Driving Large Off-Chip Loads,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/658,468, filed on Mar. 4, 2005, and entitled “Circuits and Techniques for Driving Large Off-Chip Loads,” which are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electronic circuits, and, more particularly, to a method and system to drive large off-chip loads, such as, for example, laser diodes.

2. Art Background

Nowadays, the design of modern very large scale integration (“VLSI”) systems is performed with a view towards lowering manufacturing costs. Complementary metal oxide semiconductor (“CMOS”) device technology is particularly attractive, since it is relatively low-cost, has high-performance, and enables the integration of greater device functionality on a single integrated circuit chip. Although CMOS technology scales well, some of the system blocks that interface the integrated circuit (“IC”) with external devices are difficult to integrate because the necessary output signals levels may not scale as well.

Such high integration of functionality on a single integrated circuit chip is exemplified in a typical 10-gigabit Ethernet chip, which includes an extension attachment unit interface (XAUI), a packet coding sublayer (PCS), a limiting amplifier, a clock and data recovery and demultiplexer module, as well as a 10 Gigabit/second (Gbps) multiplexer module. The 10 Gbps multiplexer module further interfaces with an external buffer that drives the inputs of an external load, such as, for example, a laser diode. The photonic emission of the laser diode is modulated, sending signals down an optical fiber. The voltage swing requirement to modulate these laser diodes may be stringent, requiring the use of a fairly large driver device.

For most applications today, a buffer device, integrated with the IC, interfaces with an off-chip laser driver. The single-ended input impedance is typically a standard value, such as 50 Ohms (50Ω). The output of the off-chip laser driver is designed appropriately for any given application in order to drive a certain diode load. For some applications, the laser diodes may be driven single-ended, with the diode biased appropriately to provide a 50Ω termination. Alternatively, the diodes may be driven differentially. For a differential configuration, the diode may be again biased to provide a 50Ω termination, which is equivalent to a single-ended termination of 25Ω.

Since the prior applications involve a high overall system cost, in part due to the cost of the off-chip driver device, what is needed is an integrated circuit that would eliminate the cost of the off-chip laser driver device by integrating an on-chip driver device capable of directly driving an external diode load. In addition, what is needed is an integrated circuit that would be capable of driving both a single-ended 50Ω load, as well as a differential 50Ω load, equivalent to 25Ω single-ended, thus allowing for greater flexibility, depending on the current application.

SUMMARY OF THE INVENTION

A method and system to drive large off-chip loads, such as, for example, laser diodes are described. The system includes an integrated circuit coupled to an external differential diode load. Alternatively, the external diode load may be driven single-ended. The integrated circuit includes a data buffer device and a clock buffer device. The integrated circuit also includes a multiplexer device coupled to the clock buffer device configured to multiplex a data input signal and a clock input signal received at respective inputs of the integrated circuit. If the external diode is single-ended, the data input signal is transmitted to the data buffer device, which is then used solely to drive the diode load. If the diode load is differential, the data buffer device receives the data input signal. At the same time, the multiplexer device receives both the data input signal and the clock input signal and selects the data signal to drive the clock buffer device. For a diode load being driven differentially, the outputs of the two buffer devices are merged together externally through the use of a power-combining network, which includes external or off-chip transmission lines, which carry a respective output signal for the data buffer device and the clock buffer device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a conventional method and system to drive a laser diode from an integrated circuit through the use of an off-chip or external driver circuit;

FIG. 2 is a block diagram illustrating a conventional matching network to drive a 50Ω differential diode load;

FIG. 3 is a block diagram illustrating a system to drive an off-chip device load using a power-combining network, according to one embodiment of the invention;

FIG. 4 is a schematic diagram illustrating a merging node of two transmission lines within the power-combining network, according to one embodiment of the invention;

FIG. 5 is a flow diagram illustrating a method to drive an off-chip device load using a power-combining network, according to one embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating a conventional method and system to drive a laser diode from an integrated circuit through the use of an off-chip or external driver circuit. In the system 100 shown in FIG. 1, the integrated circuit 101, also labeled as “Module,” is coupled to an external driver circuit 102, which drives an external laser diode 103. The input termination of the external driver circuit 102 is a predetermined impedance Z₀, such as, for example, a single-ended impedance of 50Ω. The laser diode 103 may be driven single-ended and may be biased to provide a 50Ω termination. Alternatively, in some applications, the laser diode 103 may be driven differentially, again being biased to provide a 50Ω termination. It is known that a differential termination of 50Ω is equivalent to a single-ended termination of 25Ω.

The elimination of the off-chip external laser driver device 102 and integration of its functionality into the IC 101 lowers the overall cost of the system 100. However, the two different loading conditions at the diode 103 (i.e., single-ended and differential) make it difficult for a single integrated or on-chip laser driver device to meet the matching requirement at the source end. In addition, once the output impedance of this laser driver device is set to a predetermined value, it cannot be easily changed. The use of additional programmable devices to switch the resistance may result in a significant increase in parasitic capacitance, compromising the bandwidth of the output signal.

Impedance termination of the on-chip laser driver device is particularly important, since the impedance of the laser diode 103 may not be matched to the transmission line. Even if the small-signal resistance of the laser diode 103 may be exactly set to a nominal value of 50Ω, large signals swings may cause this resistance to change significantly. Therefore, if the on-chip driver device is not terminated properly, reflections at the load end may result in significant inter-symbol interference.

FIG. 2 is a block diagram illustrating a conventional external matching network to drive a 50Ω differential diode load. In one known approach shown in FIG. 2, the off-chip driver device 102 of FIG. 1 is replaced with an off-chip or external matching network 201 to provide impedance transformation for the applications that require it, such as, for example, if the single-ended driver impedance is terminated to a nominal standard value of 50Ω. However, in this approach, a cost-benefit analysis may indicate that multi-GigaHertz (GHz) broadband, lossless matching networks may be more expensive than the off-chip driver device 102 being eliminated. In addition, the matching networks, such as the matching network 201 shown in FIG. 2, have some non-ideal properties that result in significant performance degradation (e.g., loss, relatively narrow bandwidth, and/or poor reflection coefficients). For example, if substantial loss in the matching network 201 is tolerated, then more power from the driver end is required to compensate for the loss. Hence, the driver size must also increase, resulting in additional parasitic capacitance and lower bandwidth. The increase in driver size may also make the source impedance termination requirement harder to meet.

FIG. 3 is a block diagram illustrating a system to drive an off-chip device load using a power-combining network, according to one embodiment of the invention. As illustrated in FIG. 3, the system 300 includes an integrated circuit 301 coupled to an external differential diode load 302, biased, for example, to an impedance termination value of 50Ω. Alternatively, the external diode load 302 may be driven single-ended, to an impedance termination value of 50Ω.

In one embodiment, the integrated circuit 301 further includes a data buffer device 311 and a clock buffer device 312, each having a single-ended source impedance value of 50Ω. The integrated circuit 301 also includes a multiplexer device 313 coupled to the clock buffer device 312 configured to multiplex a data input signal 321 and a clock input signal 322 received at respective inputs of the integrated circuit 301, as described in detail below.

In one embodiment, if the external diode load 302 is single-ended, the data input signal 321 is transmitted to the data buffer device 311, which subsequently is the sole device that drives the diode load 302. In an alternate embodiment, if the diode load 302 is a differential 50Ω load (25Ω single-ended), the data buffer device 311 receives the data input signal 321. At the same time, the multiplexer device 313 receives both the data input signal 321 and the clock input signal 322 and selects one of the two signals to output a multiplexed signal 323, which is then fed through the clock buffer device 312. For the differential diode load embodiment, the outputs of the two buffer devices 311 and 312 are merged together externally through the use of a power-combining network 330, which includes external or off-chip 50Ω transmission lines, which carry a respective output signal for the data buffer device 311 and the clock buffer device 312, as described in further detail below.

In one embodiment, since the multiplexer device 313 introduces an inherent delay in the system 300, the integrated circuit 301 further includes an additional buffer stage δ 314, prior to the data buffer device 311, which receives the data input signal 321 and permits equalizing the delays and signal swings of the two paths.

In one embodiment, the power-combining network 330 further includes a first transmission line pair 331, coupled to the data buffer device 311 at a first end, and a second transmission line pair 332, coupled to the clock buffer device 312 at a first end. A second end of each of the first and second transmission line pairs 331, 332 is coupled to a third transmission line pair 333 at a first end, the third transmission line pair 333 being coupled to the diode load 302 at a second end.

FIG. 4 is a schematic diagram illustrating a merging node of two transmission lines within the power-combining network 330, according to one embodiment of the invention. In the general single-ended embodiment of FIG. 4, two transmission lines 331 and 332 of respective characteristic impedances Z₁ and Z₂ merge to drive a transmission line 333 of characteristic impedance Z₃. Each incoming unit pulse on the Z₂ line 332 generates three pulses, one for each line, when incident on the merge node N, as shown in FIG. 4. One pulse, of magnitude Γ₂, is reflected back to the driver. In addition, distinct and identical pulses of magnitude 1+Γ₂ are launched on lines Z₁ 331 and Z₃ 333. Similarly, a unit pulse from the Z₁ line 331 generates a reflected pulse of magnitude Γ₁, and launches pulses of magnitude 1+Γ₂, on lines Z₂ 332 and Z₃ 333.

For no net generated pulse to exist on lines Z₁ 331 and Z₂ 332, 1+Γ₁+Γ₂=0

If Z₃=Z₁∥Z₂=Z₁Z₂(Z₁+Z₂)⁻¹, then the condition is satisfied.

For example, Z₁=Z₂=50Ω, Z₃=Z₁∥Z₂=25Ω. On line Z₃ 333, there exists a net pulse of magnitude 2+Γ₁+Γ₂=1. To observe no spurious reflected pulses, the incident pulses along lines Z₁ 331 and Z₂ 332 require exact synchronization.

In an alternate embodiment, the Z₁ line 331 and the Z₂ line 332 are virtually combined into a single line with characteristic impedance Z₁∥Z₂. If the respective pulses along these lines move exactly in step, and if the lines have identical lengths, the composite pulse moving along the composite line will arrive at the merge node N and see no difference, since the Z₃ line 333 is chosen to be Z₁∥Z₂. In this embodiment, the composite pulse continues to propagate along the Z₃ line 333, unaffected. This technique may be extended to N incoming lines at the input, merging together to drive a line with characteristic impedance Z₁∥Z₂∥Z₃∥Z₄ . . . Z_(N).

FIG. 5 is a flow diagram illustrating a method to drive an off-chip device load using a power-combining network, according to one embodiment of the invention. As shown in FIG. 5, at processing block 510, a data input signal 321 is received at one input of an integrated circuit 301. At processing block 520, a clock input signal 322 is received at a second input of the integrated circuit 301.

At processing block 530, the data input signal is transmitted to the data buffer device 311 through a delay buffer stage 314. At processing block 540, the data input signal 321 and the clock input signal 322 are received at the input of a multiplexer device 313 coupled to the clock buffer device 312.

At processing block 550, a decision is made whether the diode load 302 is driven differentially or single-ended. If the diode load 302 is driven differentially, then at processing block 560, the data input signal 321 is selected at the multiplexer device 313 and a multiplexed signal 323 is output to the clock buffer device 312. In one embodiment, the multiplexer device 313 receives the data input signal 321 and the clock input signal 322, and outputs the data input signal 321 as a multiplexed signal 323 to the clock buffer device 312.

Next, at processing block 570, a respective output signal for each of the data buffer device 311 and the clock buffer device 312 is transmitted to a power-combining network 330 to drive the diode load 302.

If, however, the diode load 302 is not driven differentially, at processing block 580, the clock input signal 322 is selected at the multiplexer device 313 and a multiplexed signal 323 is output to the clock buffer device 312. In one embodiment, the multiplexer device 313 receives the data input signal 321 and the clock input signal 322, and outputs the clock input signal 321 as a multiplexed signal 323 to the clock buffer device 312.

Finally, at processing block 590, an output signal for the data buffer device 311 is transmitted to drive the diode load 302.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention. 

1. A system for driving a device load off-chip, the system comprising: a multiplexer to receive a selection signal, a data signal, and a clock signal, the selection signal to indicate whether the device load is differential or single-ended, the multiplexer to output the clock signal if the selection signal indicates that the device load is single-ended or to output the data signal if the selection signal indicates that the device load is differential; a delay element to receive the data signal parallel to the multiplexer and to add a delay to the data signal to generate a delayed data signal; and a data buffer, coupled to the delay element, to receive the delayed data signal and to output the delayed data signal to a power network.
 2. The system of claim 1, wherein the device load comprises a laser diode.
 3. The system of claim 1, wherein the power network comprises a plurality of transmission lines to merge together the delayed data signal and the data signal outputted by the multiplexer if the selection signal indicates that the device load is differential.
 4. The system of claim 1, further comprising a clock buffer, coupled to the multiplexer, to output a multiplexer output to the power network.
 5. The system of claim 4, wherein a first path comprises a signal path through the data buffer, a second path comprises a signal path through the clock buffer, and the delay comprises an amount to equalize the delay and a signal swing between the first and second paths.
 6. The system of claim 3, wherein the power network further comprises a first transmission line pair, coupled to the data buffer at a first end, and a second transmission line pair, coupled to the clock buffer at a first end, a second end of the first and second transmission line pairs being coupled to a third transmission line pair at a first end, and the third transmission line pair being coupled to the device load at a second end.
 7. The system of claim 6, each line of the first transmission line pair comprises a characteristic impedance of Z₁, each line of the second transmission line pair comprises a characteristic impedance of Z₂, and each line of the third transmission line pair comprises a characteristic impedance of Z₃, wherein Z₁=Z₂ and ½(Z₁)=Z₃.
 8. A method for driving a device load off-chip, the method comprising: receiving a clock signal, a data signal, and a selection signal, the selection signal to indicate whether the device load is differential or single-ended; outputting the clock signal if the selection signal indicates that the device load is single-ended or outputting the data signal if the selection signal indicates that the device load is differential; delaying the received data signal by a delay to generate a delayed data signal; and receiving, by a data buffer, the delayed data signal and outputting the delayed data signal to a power network.
 9. The method of claim 8, wherein the device load comprises a laser diode.
 10. The method of claim 8, further comprising merging the delayed data signal and the data signal if the selection signal indicates that the device load is differential.
 11. The method of claim 8, wherein the outputting of the clock signal or the data signal comprises: receiving the clock signal, the data signal, and the selection signal at a multiplexer; receiving an output of the multiplexer at a clock buffer; and outputting the multiplexer output from the clock buffer to the power network.
 12. The method of claim 11, wherein a first path comprises a signal path through the data buffer, a second path comprises a signal path through the multiplexer and the clock buffer, and the delay comprises an amount for equalizing the delay and a signal swing between the first and second paths.
 13. The method of claim 10, wherein the power network further comprises a first transmission line pair, coupled to the data buffer at a first end, and a second transmission line pair, coupled to the clock buffer at a first end, a second end of the first and second transmission line pairs being coupled to a third transmission line pair at a first end, and the third transmission line pair being coupled to the device load at a second end.
 14. The method of claim 13, each line of the first transmission line pair comprises a characteristic impedance of Z₁, each line of the second transmission line pair comprises a characteristic impedance of Z₂, and each line of the third transmission line pair comprises a characteristic impedance of Z₃, wherein Z₁=Z₂ and ½(Z₁)=Z₃.
 15. A circuit for driving a device load, the circuit comprising: a multiplexer to receive a selection signal, a data signal, and a clock signal, the selection signal to indicate whether the device load is driven differentially or driven single-ended, the multiplexer to output the clock signal if the selection signal indicates that the device load is driven single-ended or to output the data signal if the selection signal indicates that the device load is driven differentially; a delay element to receive the data signal parallel to the multiplexer to add a delay to the data signal to generate a delayed data signal; and a data buffer, coupled to the delay element, to receive the delayed data signal and to output the delayed data signal to a power network, wherein both ends of the device load are coupled to the power network.
 16. The circuit of claim 15, wherein the device load comprises a laser diode.
 17. The circuit of claim 15, wherein the power network comprises a plurality of transmission lines to merge the delayed data signal and the data signal if the selection signal indicates that the device load is driven differentially.
 18. The circuit of claim 15, further comprising a clock buffer, coupled to the multiplexer, to output a multiplexer output to the power network.
 19. The circuit of claim 18, wherein a first path comprises a signal path through the data buffer, a second path comprises a signal path through the clock buffer, and the delay comprises an amount to equalize the delay and a signal swing between the first and second paths.
 20. The circuit of claim 17, wherein the power network further comprises a first transmission line pair, coupled to the data buffer at a first end, and a second transmission line pair, coupled to the clock buffer at a first end, a second end of the first and second transmission line pairs being coupled to a third transmission line pair at a first end, and the third transmission line pair being coupled to the device load at a second end. 